The invention is based on a hydraulic dual-circuit servo brake apparatus. A servo brake apparatus of this type is known (German Offenslegungsschrift No. 29 42 979).
In this known servo brake apparatus, a main cylinder piston, after executing a certain stroke, pushes open a valve which permits pressure medium to flow out of a reservoir into a brake circuit. An apparatus of this type has the disadvantage that if there is a failure of one brake circuit, the supply of pressure medium is soon exhausted. When a hydraulic booster is also connected to the system, this means that the booster will fail as well.
If a known apparatus of this type is combined with an anti-skid apparatus, then the main cylinder piston and with it the brake pedal must execute an additional stroke whenever the anti-skid means is functioning. At smaller pressure, a longer travel is necessary and with larger pressure values a shorter travel is necessary to reach the pressure supply point. Such a control means is confusing to the driver, however, and thus unsatisfactory.